1. Field of the Invention
This invention relates to anchored offshore platforms of the type used in oil and gas drilling and production and more particularly to an apparatus that forms an additional buoyant portion of a tension leg platform, located so as to reduce the pitch period of the platform. Decreasing the pitch period of the platform generally increases the fatigue life of the tension legs, and/or reduces the number and size of the tension legs.
2. Background of the Invention
In deep water, for example 800 feet or more, the installation of bottom mounted steel or concrete structures for oil well drilling and production operations becomes quite expensive due to the high cost of fabrication and installation of such large structures. A more economical solution to the problem of providing a suitable semi-permanent site for drilling and producing operations in deep water is the use of a floating structure which is moored to fixed sea floor anchor points by means of vertical tension legs. Such a structure is known as a tension leg platform.
A tension leg platform is a complex system which is difficult to design by the usual "cut and try" approach. The various elements in the tension leg platform system are strongly coupled and interact with each other in a complicated manner.
As can be imagined, design of a tension leg platform must be done with full recognition of a number of design constraints. The most important constraints to be considered include maximum stress in the legs, fatigue life of the legs (as affected by cyclic pitch and heave motions of the tension leg platform), minimum leg tension at the bottom, and maximum platform offset. Satisfaction of these constraints requires analysis of the response of the tension leg platform to winds, waves, and current.
Estimation of the fatigue life or required strength in the legs involves consideration of the so-called "pitch period criterion" which is a relation between the maximum recommended resonant or natural pitch period of the tension leg platform versus the platform's installed water depth. The pitch period criterion is really a derived criterion based upon consideration of the fatigue life of the tension legs. It represents an attempt to alter the tension leg platform motion response characteristics with respect to prevailing wave energy such that a resonably long fatigue life of the tension legs can be assured. In general, the pitch period for a tension leg platform should be designed to be as low as possible or at least with a period that is not near the period of any significant energy in the sea spectra. For example, calculations indicate that decreasing the pitch period of a tension leg platform from 4.1 to 4.0 seconds may increase the fatigue life of the tension legs by approximately 33%.
Accordingly, it is desirable to present a method of reducing the pitch period response of a particular tension leg platform in order to construct or modify the design of the platform such that the pitch period response is reduced below the maximum wave spectrum frequency response areas. It is also desirable to present an apparatus that allows the pitch period response of existing tension leg platform designs to be modified sufficiently to maximize the life of the tension leg platform structure.